Workers IO runs an entire software system, including its dependencies, through millions of simulated production scenarios to find the interleavings, faults, and timings that break properties a team has defined. It is aimed at groups that already treat 99.99% uptime as the baseline, and at teams whose coding agents now land changes faster than people can review them. Founder Chaitanya Choudhary is building the company in San Francisco as part of Y Combinator's Fall 2026 batch. The launch is pointed at the gap described in the announcement: agents can produce a large amount of code in a short time, while verifying that code under realistic failure still takes significant engineering work. The simulator executes the system as one deterministic timeline inside virtual machines, then branches that timeline across many seeds. It injects faults such as network delays, disk errors, clock skew, and process failures, and a virtual clock skips real waiting so timeouts and long-running behavior can be explored without wall-clock delay. Teams state the promises that must hold. The product demo uses checks such as whether money stays accounted for while a payment provider is unreachable, and whether a retry charges a card only once. A failed seed can be reproduced with the same faults, the execution trace, and the return values of each dependency. An engineer or a coding agent can narrow that case to a cause, apply a fix, and rerun the same conditions to confirm the promise holds before release. Each change an agent proposes can be checked against those scenarios, with a broken promise returned as a replayable case.